Electrophotographic imaging processes have been extensively described in both the patent and other literature, for example in U.S. Pat. Nos. 4,514,481; 4,471,039; and 4,175,960 (and patents cited therein). Generally, these processes have in common the steps of electrostatically charging an electrophotographic element and exposing the element imagewise to electromagnetic radiation, thereby forming an electrostatic charge image. A variety of subsequent operations, well known in the art, can then be employed to produce a permanent record of the image.
Electrophotographic elements comprise a conducting support bearing a layer of a photoconductive material which is insulating in the dark but which becomes conductive upon exposure to activating radiation. A common technique for forming images with such elements is to uniformly electrostatically charge the surface of the element and then imagewise expose it to activating radiation. Upon exposure of an electrostatically charged electrophotographic element, electron-hole pairs are formed in the electrophotographic layer of the element. When the element is electrostatically charged with a negative potential, the hole migrates to the surface of the element, thereby dissipating the surface charge in the exposed areas. When the electrophotographic element is charged with a positive potential, the electron migrates toward the surface of the element, thereby dissipating the surface charge imagewise in the exposed areas.
Left behind is a charge pattern referred to as a latent electrostatic image, which can then be developed, either on the surface on which it is formed, or on another surface to which it has been transferred, by application of a liquid or dry developer composition which contains finely divided electroscopic marking particles, known as toner particles. These particles are either selectively attracted to and deposited in the charged areas, or are repelled by the charged areas and selectively deposited in the uncharged areas. The pattern of marking particles can be fixed to the surface on which they are deposited or it can be transferred to another surface and fixed there.
Electrophotographic elements can comprise a single active layer, containing the photoconductive material, or they can comprise multiple active layers. Elements with multiple active layers (sometimes referred to as multi-active elements) have at least one charge-generation layer and at least one charge-transport layer. The charge-generation layer responds to activating radiation by generating separated electron-hole pairs, one member of which pair (the one of opposite sign to the surface charge) migrates to the surface as a result of the attraction of the surface charge, and there neutralizes that charge. A latent electrostatic image is thus produced on the surface. The remaining member of the electron-hole pair then goes to ground. Multi-active elements are extensively described in U.S. Pat. No. 4,175,960, which is hereby incorporated by reference.
Numerous photoconductive materials have been described as being useful in electrophotography. These include inorganic materials, the best known of which are selenium and zinc oxide, as well as monomeric and polymeric organic materials, such as arylamines, arylmethanes, azoles, carbazoles, pyrroles, phthalocyanines, polyvinylcarbazoles, and the like.
Typically, monomeric charge-transport materials have been incorporated in an electrophotographic element in solid solution in a polymer binder matrix. Alternatively, amorphous polymeric charge-transport materials have been used. In either case, the individual molecules of the charge-transport material are randomly oriented, which limits the mobilities of the charges and thus the speed with which the charges reach the surface of the element.
As described by Gray (G. W. Gray, ed., "Thermotropic Liquid Crystals" in Critical Reports on Applied Chemistry, Vol. 22, Wiley 1987, p.x), liquid crystalline compounds can be divided into two general classes: lyotropic liquid crystals, which are ordered arrangements of micelles that arise in solutions of amphiphilic compounds at intermediate concentrations; and thermotropic liquid crystals, which are formed from rigid molecules, usually organic and generally either rod-shaped or disc-shaped, either by heating the crystalline solid or cooling the isotropic liquid.
As reported by H. Finkelmann et al. (Makromol. Chem., 1978, 179, pp. 273-276), the mesomorphic properties of low molecular weight liquid crystalline compounds are also observed in macromolecules; polymeric materials that contain rigid mesogenic groups attached by flexible spacer groups to backbones generally do not crystallize at lower temperatures but exhibit glass transition temperatures at which liquid crystalline phases are formed.
L. L. Chapoy et al. (Macromolecules, 1983, 16, pp. 181-185) proposed that electronic transport phenomena such as photoconductivity could be enhanced if the active groups were contained in a polymer possessing long-range molecular order and, with this in mind, prepared a polylysine helical polymer containing carbazole substituents. It was reported that no thermotropic liquid crystalline behavior was observed with this material but that in concentrated solution it formed a cholesteric lyotropic mesophase.
There is a need for photoconductive materials that exhibit improved charge-carrier mobility. The present invention provides novel hole-transport compounds with highly ordered molecular structures.